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The discovery was made from images taken by the interplanetary spacecraft Voyager 2 in late 1986.

Although the images have been publicly available, no-one recognised the new moon until the recent investigation of the pictures by a researcher from Arizona University, Erich Karkoschka.

"Typically satellites [moons] are found within days after the discovery image has been taken," he said. "In this case, the image is more than 13 years old."

Until now, Saturn has been the only planet in our solar system known to have as many as 18 moons, although Jupiter comes close with 16.

The finding dispels theories that smaller planets have less moons than larger ones. In fact, Karkoschka believes there may well be more undiscovered moons around Uranus because the moons known so far are twice the size of those around Jupiter and Saturn, and dim sunlight would make it hard to see smaller objects in Uranus's atmosphere.

Based on a detection in seven images, he concluded that the newly-discovered moon orbits Uranus once every 15 hours and 18 minutes, at an altitude of about 51,0000 kilometres - roughly the same distance as the planet's diameter.